This invention relates generally to battery packs and more particularly to testing methods for determining the condition of a battery cell or weld.
The battery assembly for hybrid or plug-in electric vehicles (EV) may consist of a plurality of battery cells which are stacked and joined together to form a module or pack. The battery for a long driving range EV may contain more than 200 battery cells. Typically, after stacking and assembling the battery components into a module, every two or three adjacent battery cells are welded together to form parallel electric connections. Each cell has at least two tabs or electric terminals (one positive, one negative) for such welding. An interconnect board can be added and welded to the cells to complete the serial connection of the battery pack, if desired. The welding operations require proper welding machines and tooling, and they are expensive and time consuming operations. Improper welding can lead to problems with the battery pack in use.
Methods of testing weld and cell integrity have been developed. For example, the individual cell voltage has been measured, as well as the voltage range across a module, section, or pack at a particular time during a battery discharge cycle. However, these methods have not conclusively identified problems with external welds or cells. For example, a cell can be out of balance in a pack, but still be a quality part. Simple balancing techniques can be used to resolve these issues. However, if the imbalance is due to a weld problem, then the module would have to be scrapped.
In addition, a problem can occur when cells charge and discharge at different rates under different dynamic conditions. Increased temperature or significant temperature rises in the pack can be caused by improper charging or discharging, leading to lost capacity and damage to the cell. This is often an impedance issue either within the cell or in an external weld to the interconnect board (ICB).